What Color Will I Get? - Part I

Color Genetics - Part I

Thursday 19 January 2012

Whether it is your first time breeding, or you have had many foals at your farm one of the most exciting things is not knowing exactly what you will get when the baby arrives. There are so many variables that it is impossible to predict how big your foal will be, whether it will have good conformation or whether it will have the potential for the discipline you bred it for. One question that always has owners guessing is what color the foal will be.

Interestingly enough, you can actually predict the likelihood of the foal turning out a specific color. While color prediction is not always 100% accurate, there are some color combinations that will give you a guaranteed outcome. How can we be so sure of this? It’s all in the genes.

Horses have two basic color genes; black (B) and red (b). All horses carry a combination of these two genes in a pair. The red gene is recessive to the black gene. This means that whenever you get a combination of black (B) and red (b) genes the black gene will express itself. So, a (BB) gene combination will result in a black horse; a (Bb) combination will also result in a black horse and a (bb) combination will result in a red horse.

If that is the case you may wonder why we don’t only have black and red horses. In addition to the base color genes there are special genes that modify color. If one of these genes are present it will cause the base color to be modified and the horse will turn out “a different color”.

The most common modifier is the Agouti gene. When the Agouti gene is present it modifies the black gene. A black horse with the Agouti modifier will fade leaving its points black and its body brown. This is of course the classic bay horse.

Other modifiers include the grey gene, the roan gene and the cream gene. Each of these genes causes the horse’s coat to change according to the instructions written into the DNA code. Because this complicates matters for now lets stick with the basic black (bay) and red genes. To simplify things for now I will use the term “black” to include bay horses. We will discuss the Agouti gene further in the next installment.

So how can you apply this to actual breeding? Let’s start with a simple scenario. You have a chestnut stallion (bb) and you breed it to a chestnut mare (bb). In this case both the stallion and the mare have two red genes. No matter which way you mix and match those genes the resulting foal will also have two red genes (bb). The resulting foal from this breeding will always be a chestnut.

To make things a little more complicated we will now breed a black mare (BB) to a chestnut stallion (bb). Because both the mare and the stallion must contribute one gene the resulting foal will be (Bb) carrying one black and one red gene. The foal will be black in color but will have a recessive red gene.

Now let’s pretend that we breed the foal from the first scenario (bb) to the foal from the second scenario (Bb). Use the following diagram to see how the genes relate:

As you can see from the diagram there are two color possibilities in this cross. When the foal receives both a red gene (b) from one side and a black gene (B) from the other, the foal will be born black (Bb). This means that you have a 50/50 chance of the foal being a chestnut or a black.

If you cross a homozygous (meaning that it has two copies of the B gene) black horse (BB) with a heterozygous (meaning that it has one copy of the B gene and one of the b gene) black horse as you might expect there would only be one possible color for the foal, black. This is because all of the color combinations would include at least one dominant black (B) gene.

Of course this leaves one more combination, the (Bb) x (Bb) combination. In this case both the mare and stallion carry a recessive red gene but are black in color. While at a glance you might expect all the foals from this breeding to be born black this is where you get the odd surprise foal that is a bright chestnut. In fact 25% of all foals from this cross will be chestnut in color. That is because of the possibility of the foal getting a pair of red genes, one from each parent.

So now you can see where the basics of color genetics come into play when breeding horses. You now know that you will always get a chestnut foal when you breed two chestnut horses together. You also know that when you breed two blacks together you just might get the odd chestnut foal that crops up.

While color is far from the most important thing to consider when breeding, it is certainly one of the most predictable. With the advances in genetic research we are always finding more color specific genes and can predict color variations with increasing accuracy.